SAR as such is a known technique, by which it is possible with a radar system mounted on a moving platform, normally an aircraft or a satellite, to obtain high resolution images of the ground. Radar responses from the ground are stored during some interval of the flight of the platform. The SAR image is obtained by signal processing in ways similar to computer tomography. Image resolution is determined by the angular span of viewing angles of the imaged ground, as well as the wavelength used and the distance between the radar and the ground. This means that the actual resolution of the radar antenna is of no importance for the resolution of the obtained image.
In SAR there is no fundamental requirement on resolution of the radar antenna, which consequently is not required to be directive. Thus it is possible to use unconventional radar frequencies, like meter waves, for SAR. For radar applications, which require a directive antenna, and use microwave frequencies, an antenna dish of around one meter is typical. By analogy, using meter waves, an antenna dish around hundred meters would be required. A small meter wave antenna would invariably produce a non-directive dipole radiation. For meter wave SAR such antenna is feasible since antenna resolution is of no importance.
There are SAR radar systems operating over the frequency band 27.5-82.5 MHz, corresponding to wavelengths between 11 m to 3.5 m, using this type of dipole antenna. Such a SAR radar can obtain images of the ground with a resolution of around 2.5 meters and operates across a frequency band of more than one octave. High radiation efficiency is achieved by letting the dipole antenna be of a length of around half the mean wavelength of the frequency band and also possessing significant thickness to provide sufficient bandwidth of the antenna. Suitable dimensions for such an antenna are a diameter of 0.2 m and a length of 4 m. Antennas of these dimensions are quite feasible on midsized and larger aircraft.
Several important SAR applications however, require small platforms such as small manned, fixed wing or rotary wing aircrafts or tactical Unmanned Aerial Vehicles (UAVs), for which antennas with the stated type of dimensions are too large.
U.S. Pat. No. 5,900,843 B discloses a VHF antenna for airborne SAR. The antenna is formed by cutting a slotline in the middle of the top wall of a very thin waveguide along its axis. The antenna is said to produce a downward and side-looking beam with horizontal polarization. The antenna dimensions required for this solution at VHF frequency range are however of the order of several meters and not feasible to use at small platforms.
There is thus a need to achieve smaller SAR antennas suitable for small manned vehicles and UAVs without compromising the SAR image quality.